1. Field of the Invention
The present invention relates to a light-emitting device which is constricted forming an EL (electro-luminescent) element on a substrate, the EL element being such that a light emissive material (herein below, termed xe2x80x9cEL materialxe2x80x9d) which can emit the light of fluorescence or phosphorescence by applying an electric field is sandwiched between a non-transparent electrode (cathode) and a transparent electrode (anode). Concretely, the invention relates to enhancement in the efficiency of deriving light from an EL element.
By the way, in the invention, the xe2x80x9clight-emitting devicexe2x80x9d shall signify an image display device or a luminescent device which employs the EL element. Besides, the xe2x80x9clight-emitting devicexe2x80x9d shall cover all of a module in which a connector, for example, an anisotropic conductive film (FPC: Flexible Printed Circuit), a TAB (Tape Automated Bonding) tape or a TCP (Tape Carrier Package) is attached to the EL element, a module in which a printed wiring board is disposed at the end of a TAB tape or a TCP, and a module in which an IC (integrated circuit) is directly mounted on the light emitting element in accordance with a COG (Chip On Glass) scheme.
2. Description of the Related Art
In recent years, techniques for forming TFTs (thin film transistors) on substrates have made great progress, and display devices (light-emitting devices) of active matrix type applying the TFTs have been being developed. Especially, a TFT employing a poly-silicon film exhibits a field effect mobility (also, simply termed xe2x80x9cmobilityxe2x80x9d) higher than that of a TFT employing a conventional amorphous silicon film and is capable of higher speed operation. Therefore, the control of pixels having been performed by driver circuits outside a substrate before can be performed by driver circuits formed on the same substrate as that of the pixels.
With such a light-emitting device of active matrix type, various advantages, for example, curtailment in a manufacturing cost, miniaturization in an electro-optic equipment, enhancement in an available percentage and increase in throughput, are attained by forming various circuits and elements on an identical substrate.
Further, researches have been vigorously made on light-emitting devices (EL displays) of active matrix type each having EL elements as spontaneous emission type elements.
Here in this specification, in the EL display being an example of the light-emitting device, the EL element has a structure in which an EL layer is sandwiched between a pair of electrodes (an anode and a cathode), and in which the EL layer has a multilayer structure ordinarily. Typically mentioned is the multilayer structure of xe2x80x9chole transporting layer/light emitting layer/electron transporting layerxe2x80x9d proposed by Tang et al., Eastman Kodak Company. The multilayer structure exhibits a very high emission efficiency, and most of the EL display devices being currently under researches and developments adopt this structure.
Alternatively, the multilayer structure may be so formed that the anode is successively overlaid with a hole injecting layer/hole transporting layer/light emitting layer/electron transporting layer, or a hole injecting layer/hole transporting layer/light emitting layer/electron transporting layer/electron injecting layer. The light emitting layer may well be doped with a fluorescent coloring matter or the like.
In this specification, all layers interposed between the cathode and the anode shall be generally called the xe2x80x9cEL layerxe2x80x9d. Accordingly, the hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer and electron injecting layer mentioned above are all included in the xe2x80x9cEL layerxe2x80x9d.
Herein, a predetermined voltage is applied to the EL layer of the above structure by the pair of electrodes, whereby light is emitted by the recombination of carriers taking place in the light emitting layer. By the way, in this specification, a light emitting element which is formed of the anode, EL layer and cathode shall be called the xe2x80x9cEL elementxe2x80x9d.
The EL layer included in the EL element is more deteriorated by heat, light, moisture, oxygen, etc. In general, therefore, the fabrication of the active matrix type EL display proceeds in such a way that EL elements are formed after wiring lines and TFTs have been formed in a pixel portion.
Besides, after the formation of the EL elements, a substrate provided with the EL elements (an EL panel) and a cover member are stuck together so as to prevent the EL elements from being exposed to the open air, and the resulting structure is sealed (packaged) by a sealing member or the like.
After air-tightness has been enhanced by the treatment of the packaging or the like, a connector (such as FPC or TAB tape) by which terminals led out of the elements or circuits formed on the substrate are connected with external signal terminals is attached, whereby the active matrix type EL display is finished up.
Recently, EL display devices each having EL elements have been developed. The EL element is a spontaneous emission type element of current drive type which utilizes light generation based on the recombination of electrons and holes injected into an EL layer from electrodes at both the surfaces of the EL element by applying a voltage, and from which emitted light is derived as planar one. However, a light deriving efficiency in the case where the light is emitted in the EL layer is derived as the planar emitted light out of the EL element is very low and is usually 20% or below.
Besides, the light emitted in the EL layer is wave-guided within the transparent electrode, depending upon the angle of incidence of the light. The light thus wave-guided is termed as the xe2x80x9cwave-guided lightxe2x80x9d. Part of the wave-guided light is absorbed to disappear, while the remainder is propagated within a solid thin film forming the transparent electrode, to escape to the end faces of the transparent electrode. In each pixel, therefore, the wave-guided light can be derived only partially as the planar emitted light, and light leakage to the adjacent pixel occurs in some cases.
It is accordingly an object of the invention to provide a structure in which light emitted in an EL layer is prevented from escaping from the end faces of a transparent electrode, thereby to enhance the efficiency of deriving the light and to solve the problem that an image blurs due to light leakage to an adjacent pixel.
The construction of the invention is shown in FIG. 1. By the way, a light-emitting device in the invention is such that a plurality of pixels are formed in the shape of a matrix within a pixel portion, and that each pixel includes a light emitting element 104 composed of a transparent electrode 101, an EL layer 102 and a non-transparent electrode 103 and is connected through wiring with a thin film transistor (TFT) (not shown) for driving the light emitting element 104. The xe2x80x9cwiringxe2x80x9d here signifies one which is made of a conductive material in order to establish electrical connection.
When carriers are respectively injected into the EL layer 102 from the transparent electrode 101 and the non-transparent electrode 103 formed on an insulating surface 100, they are recombined in the EL layer 102, thereby to emit light. Incidentally, the xe2x80x9ctransparent electrode 101xe2x80x9d signifies an electrode which can transmit the light (visible light) emitted in the EL layer 102 and which is therefore disposed on a side where the light is emitted. In contrast, the xe2x80x9cnon-transparent electrode 103xe2x80x9d signifies an electrode which cannot transmit the light and which is therefore disposed on a side where the light is not emitted.
Herein, a light shield portion 105 is interposed between the transparent electrodes 101 included in adjacent pixels, in order that the light emitted in the EL layer 102 in the pixel 1 may be prevented from leaking to the pixel 2 or pixel 3 which adjoins the pixel 1. Incidentally, the light shield portion 105 which is formed on this occasion may lie in touch with the peripheral part of the transparent electrode 101, but it need not always lie in touch.
Besides, the xe2x80x9clight shield portion 105xe2x80x9d in the invention signifies a portion which is formed on the insulating surface 100 overlaid with the transparent electrode 101, and which has the function of reflecting the light emitted in the EL layer 102 of the EL element 104 or intercepting it so as not to be wave-guided within the transparent electrode 101 and to leak from the peripheral part of the transparent electrode 101. Incidentally, the peripheral part of the transparent electrode 101 may lie either in touch with the light shield portion 105 or out of touch therewith.
The xe2x80x9clight shield portion 105xe2x80x9d in this specification is made of a light-shielding material which intercepts light. Such a material may be either a conductive material or an insulating material. Incidentally, the xe2x80x9cconductive materialxe2x80x9d here signifies a material which has a certain degree of electrical conductivity, and a material whose electrical conductivity is sufficiently low and which is classified as an insulator is distinguished from the conductive material as the xe2x80x9cinsulating materialxe2x80x9d.
By the way, in case of employing the conductive material for the light shield portion 105, a wiring line for connecting the transparent electrode 101 with a TFT can function as the light shield portion 105.
The conductive material which is employed for the light shield portion 105 should preferably be a material of high reflectivity. Incidentally, the xe2x80x9creflectivityxe2x80x9d in this specification signifies a proportion which the energy of reflected light occupies relative to that of light (incident light) entering the surface of the particular material, in the visible light region of radiation. The xe2x80x9cmaterial of high reflectivityxe2x80x9d signifies a material whose reflectivity is at least 60%, preferably at least 80%. Concretely, it signifies a material such as Ag, Al, Ta, Nb, Mo, Cu, Mg, Ni or Pb.
On the other hand, in a case where the light shield portion 105 is formed using the insulating material, it is possible to employ a material such as polyimide, polyamide, an acrylic resin or BCB (benzocyclobutene) in which a black pigment or carbon is dispersed. Besides, in this specification, the xe2x80x9cEL element 104xe2x80x9d signifies an element which has a structure including the non-transparent electrode 103 made of a non-transparent electrode material, the transparent electrode 101 made of a transparent electrode material, and the EL layer 102 sandwiched between the electrodes 103 and 101. The structure of the EL layer 102 may be formed only of a light emitting layer for offering the place of the recombination, or it may well include an electron injecting layer, an electron transporting layer, a hole transporting layer, an electron blocking layer, a hole blocking layer, or/and a hole injecting layer as may be needed. That is, in this specification, the xe2x80x9cEL layerxe2x80x9d includes all the layers into or within which the carriers from the electrodes are injected, transported and recombined.
Owing to the provision of the light shield portion 105, the light which progresses from the transparent electrode 101 toward the light shield portion 105 is intercepted to be reflected or absorbed. Thus, the direction of the light is controlled.
As described above, according to the invention, the light shield portion 105 is formed at the peripheral part of the transparent electrode 101 constituting the EL element 104, whereby the light emitted in the EL layer 102 can be prevented from being lost or leaking to the adjacent pixels on account of escaping from the peripheral part of the transparent electrode 101, and the efficiency of deriving the light can be sharply heightened.
Further, it should be noted that the light-emitting devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission devices, for example.